(1) Field of the Invention
The present invention relates to an optical fiber amplifier operable in the 1.5 .mu.m band, and more particular to an optical fiber amplifier which is provided with a gain equalizing circuit.
(2) Description of the Related Art
An erbium (Er)-doped fiber amplifier can be adopted widely in a variety of applications including optical booster amplifiers, optical relay devices and optical preamplifiers due to its high saturation output and high gain characteristics and due to polarization non-dependency characteristics. Thus, the extent to which the Er-doped fiber amplifier can be used in optical communication has expanded remarkably. An example in which the Er-doped fiber amplifier is used in the optical communication has been disclosed by K. Hagimoto, K. Iwatsuki, A. Takada, M. Nakazawa, M. Saruwatari, K. Aida, K. Nakagawa and M. Horiguchi under the title "A 212 km non-repeated transmission experiment at 1.8 Gb/s using LD pumped Er3.sup.+ -doped fiber amplifiers in an IM/direct detection repeater system", presented at OFC 89, PD-15 (January 1989). S. Shimada has written about "Impact of the Er-doped fiber amplifier on optical communication by the Er-doped fiber amplifier" in O plus E, 113, p. 75 (April 1989). Furthermore, K. Nakagawa, S. Nishi, K. Aida and E. Yoneda have written about "Trunk and distribution network application of erbium-doped fiber amplifier", IEEE J. Lightwave Technology, Vol., 9, No. 2, pp. 198-208 (February 1991).
However, when applied to wavelength-multiplexed communication systems, the optical signal in which the wavelength at the gain is relatively low degrades severely due to the signal wavelength dependency characteristics of the gain of the Er-doped fiber amplifier. As an example of a means to solve this problem, a method for setting a signal bandwidth for each signal beam proportionally to a gain of each wavelength based on the gain bandwidth characteristics of an optical fiber amplifier has been proposed in Japanese Patent Application Kokai Publication No. Hei 03-089644 (Apr. 15, 1991). Also, a method for making use of a wavelength-multiplexed transmission system or an optical type Mach-Zehnder equalizer has been disclosed by H. Toba, K. Nakanishi, K. Oda, K. Inoue and T. Kominato under the title "A 100-Channel Optical FDM Six-Stage In-Line Amplifier System Employing Tunable Gain Equalizers" in IEEE Photonics Technology Letters, Vol. 5, No. 2, pp. 248-251 (February 1993).
However, where multi-stage amplification is performed, for example, it is difficult to compensate for accumulated gain unbalance or it becomes costly to fabricate a gain equalizer and, where a fiber type Mach-Zehnder gain equalizer is used, there is a problem associated with speed of response since the wavelength-transmittance characteristics are controlled by means of temperature control of the fiber. Thus, no ideal solution to these problems has been available. Also, with respect to analog optical transmission, it has been reported that, due to the signal wavelength dependency of gain of an Er-doped fiber amplifier, a compound secondary distortion occurs, resulting in the degradation of transmission signals. In this relation, there is a report on the compensation of gain using an optical bandpass filter in which the tuned wavelength of the filter is slightly deviated from the wavelength of the optical signal (by K. Kikushima, under the title "Using Equalizers to Offset the Deterioration in SCM Video Transmission due to Fiber Dispersion and EDFA gain tilt" in IEICE, 1992, B-705). However, this cannot be used where the signal is of multi-wavelengths.
Therefore, in order to solve the above problems, there is a need for the development of a technique for the compensation of the gain in the Er-doped fiber amplifier.